The present invention relates to a direct access storage device (DASD), and more particularly to an unconstrained equalization method and apparatus with gain and timing bias to control runaway for a direct access storage device (DASD) data channel.
In today""s disk drives the readback signal is equalized using a finite impulse response (FIR) filter. There are numerous algorithms used to find the FIR tap weights that will result in giving the optimum equalization. In all the algorithms, the FIR taps must have their gain and phase shift constrained during the equalization process in order to prevent interactions with the channel gain and timing loops. For example, the gain and timing loops inputs come from the output of the FIR. If the FIR gain was not constrained then if its gain were to begin increasing during the equalization process the channel gain loop would see an increasing signal amplitude and begin reducing the incoming data signal. Thus the FIR gain and the channel gain loop would be fighting one another. The same problem would exist for the timing loop. The standard method used to avoid this problem is to constrain the tap weights either by holding some taps at a constant value or to keep the sum of the even and the sum of the odd taps each equal to some specified values.
These constraints prevent the FIR from delivering the optimum equalization.
A need exists for a method for preventing the gain and timing loops from fighting with the equalization algorithm and that also allows the FIR tap weights to be unconstrained. A need exists for a resulting equalization algorithm that can find the optimum set of FIR tap weights.
A principal object of the present invention is to provide an unconstrained equalization method and apparatus with gain and timing bias to control runaway for a direct access storage device (DASD) data channel. Other objects are to provide such equalization method and apparatus substantially without negative effects, and that overcome many of the disadvantages of prior art arrangements.
In brief, an unconstrained equalization method and apparatus with gain and timing bias to control runaway are provided for a direct access storage device (DASD) data channel. A direction for moving each of a plurality of tap weights of a finite impulse response (FIR) filter is calculated. New tap weights are determined for each of a plurality of tap weights in the direction. A gain of the FIR filter for the new tap weights is compared with a set threshold gain value. A polarity of a gain bias is set to adjust a gain of a gain loop of the data channel by a preselected amount responsive to the compared gain and set threshold gain values.
In accordance with features of the invention, a phase lag of the FIR filter for the new tap weights is compared with a set threshold phase lag value. A polarity of a timing bias is set to adjust a phase lag of a timing loop of the data channel by a preselected amount responsive to the compared phase lag and set threshold phase lag values.